The invention relates to lithography and, more particularly, to a mask for extreme ultra violet lithography (EUVL) and a method of fabricating the same, and to a wafer exposure method using the same.
As the circuit CD (Critical Dimension) of semiconductor devices has been reduced, EUVL has been developed to transfer a pattern having the resulting finer line width on a wafer. EUVL is anticipated to be the next generation technique to fabricate a smaller and faster microchip having the CD of less than approximately 32 nm. Wavelengths of lights used in EUVL typically are less than approximately 13 nm. Such light is supposedly effectively reflected by a reflector having approximately fifty double layers. One double layer comprises a molybdenum (Mo) layer and a silicon (Si) layer, each of which has a thickness of approximately 7 nm and is deposited with almost atomic scale accuracy.
Because light with a relatively short wavelength is used in EUVL, a mask structure, in which a circuit pattern to be transferred on a wafer is prepared as a mask pattern, includes a light reflective structure rather than a light transmissive structure, as used in other mask structures. An EUVL mask is fabricated by incorporating a light reflective layer and a light absorption layer pattern into the mask. The light reflective layer includes a multi-layered structure of an Mo/Si layer on a transparent substrate like a quartz substrate, and the light absorption layer pattern is formed on the light reflective layer to expose a portion of the light reflective layer. Accordingly, the light absorption layer pattern is formed according to a layout of a pattern that will be transferred on the wafer.
In this EUVL mask, the height of the light absorption layer pattern is different from that of the light reflective layer disposed below the light absorption layer. During an EUVL exposure process, an exposure light is incident to and reflected by the surface of the EUVL mask at an inclined incident angle rather than an angle perpendicular to the surface of the EUVL mask. However, because the light absorption layer pattern protrudes from the light reflective layer, incident light or reflected light can be screened by the edge of the light absorption layer pattern. Moreover, diffused reflection can occur on the light absorption layer pattern through incident light or reflected light. As a result, exposure failure may occur such that an inaccurate pattern image will be transferred on a wafer.
Furthermore, while performing an exposure process through the EUVL mask, incident light direction is adjusted to allow exposure light, i.e., extreme ultraviolet light, to be incident to and reflected by the surface of a substrate where a light reflective layer and a light absorption layer pattern are formed. Accordingly, if another passivation layer or film is introduced to protect a light reflective layer or a light absorption layer pattern, unwanted light absorption may occur due to the passivation layer or film. Accordingly, because there is a limitation in introducing a passivation layer or film, defects may occur on the surfaces of a light absorption layer pattern or a light reflective layer. This may serve as a factor for flawed pattern transfer.